In f73f0c7b (git_access: Use batched new blobs check, 2021-06-21), a
new feature flag had been added to enable batched computation of changes
size in Git authentication checks. With this feature flag enabled, Git
access checks compute all new blobs with a single RPC call instead of
doing one call per changed reference. Given that these RPCs are rather
expensive, this feature flag improves performance when multiple changes
are checked in a context where no quarantine directories are available.

This feature flag has been tested in production since June 22nd, but
couldn't yet be default-enabled because the RPC it relies on has only
been released with v14.1. Given that the feature has been tested for
such a long time now, let's remove its feature flag altogether.

Changelog: performance